Three types of copying materials are known which utilize the light-sensitivity of diazo compounds. One is a wet-process development type copying material in which a light-sensitive layer comprising mainly a diazo compound and a coupling component is provided on a support and in which development is carried out by using an alkaline solution after imagewise exposing the material superposed with an original. The second type is known as a dry-process development type copying material in which development is carried out by using ammonia gas, unlike the wet-process copying material. The third type is known as a heat-developable copying material. Examples of the heat-developable copying material include a type in which its light-sensitive layer contains an ammonia gas-generating agent (e.g., urea) capable of generating ammonia gas upon heating, a type in which its light-sensitive layer contains an alkali metal salt of an acid (e.g., trichloroacetic acid) having such a property that the acid loses its acidic property upon heating, and a type in which a higher fatty acid amide used as a color forming aid is melted by heating to activate a diazo compound and a coupling component.
The wet-process copying material has some problems because developing solutions are used. For example, much labor for supplementing or disposing of the solution is required. Apparatuses are large so that there is a problem in the maintenance thereof. Copies immediately after copying are wet and hence retouching can not be immediately done. Also, the copied image can not be preserved over a long period of time.
The dry process copying material requires supplementing the developer as in the wet-process copying material by providing a gas absorption equipment so that evolved ammonia gas does not escape. Therefore, there are the problems that apparatuses are large-sized and there is odor of ammonia ga immediately after copying.
The heat developable copying material has the merit that no developing solution is used, unlike the wet-process copying material and the dry-process copying material. However, conventional heat-developable copying material has problems, such as the need of a high developing temperature as high as 150.degree. to 200.degree. C. Further, underdevelopment or change of color tone occurs unless the temperature is controlled within .+-.10.degree. C., and hence the equipment cost is increased. Diazo compounds for use in such high-temperature development must have high heat resistance, but such compounds have often a disadvantage in the use thereof at a high density. Attempts have been made to conduct low-temperature development (90.degree. to 130.degree. C.). However, the low-temperature development has the defect that the shelf life of the copying material itself is lowered.
As described above, although the heat-developable copying material has merits as compared with the wet-process type and the dry-process copying materials, the heat-developable copying material has not been accepted as the main stream of diazo type copying systems as yet.
In order to obtain a desired color density by heating a heat-developable copying material comprising a layer containing a diazo compound, a coupling component and a basic substance, it is necessary that these components are instantaneously melted, diffused and reacted to form a colored dye. When the system is basic during the reaction, an effect of accelerating the reaction can be obtained. Accordingly, a basic substance must be incorporated in the layer to prepare a heat-developable copying material exhibiting a copying rate suitable with low-temperature heat development.
On the other hand, a possibility that the background of the heat-developable copying material is colored during storage before copying should be inhibited, or color density is lowered.
Attempts have been made to prepare a heat-developable copying material having good shelf life as well as high copying speed. However, a material has not been achieved which can be put to practical use.
When a material is so designed that color formation is sufficient even at a low heating temperature to give a high density, there is a possibility that a coloring reaction takes place even during the course of storage at room temperature before copying and as a result, the background which must be white is colored.
The present inventors have made studies to solve the above problems, which are apparently not eliminated in a simultaneous manner. They have found that one fundamental solution is to enclose a diazo compound in microcapsules in the copying material comprising a heat-developable light-sensitive layer containing the diazo compound, a coupling component and a basic substance provided on a support. However, though the background of the material can be somewhat prevented from being colored during the course of storage before copying, but insufficiently, the problem of lowering in color density is left still unsolved. Further, with a copying material containing a diazonium salt enclosed in microcapsules, its light-sensitive layer is opaque and copy can not be used as the original drawing in the reproduction of drawings for which the copying material is frequently used.
There are known many methods for producing microcapsules containing hydrophobic liquids. Examples of the methods include the following methods.
(1) Phase separation methods from aqueous solution (U.S. Pat. Nos. 2,900,457 and 2,800,458), which are the most popular methods and put into practical use and utilize the coacervation of hydrophilic colloidal sol.
(2) Interfacial polymerization methods (JP-B-38-19574 (the term "JP B" as used herein means an "examined Japanese patent publication"), JP-B-42-446, JP-B-42-771, British Patents 989,264, 950,443, 867,797, 1,069,140 and 1,046,409) in which a monomer component or a precondensate thereof as a first shell forming material is allowed to exist in an oily liquid to be encapsuled, a second shell forming material having a group capable of reacting with the first shell forming material is dissolved in a polar solvent immiscible with aforesaid oily liquid, and the first shell forming material is polymerized with the second shell forming material at the interface between the oil droplets of aforesaid oily liquid and aforesaid polar solvent to form a shell, without using a polymer as a shell material for the microcapsules from the beginning.
(3) A method for polymerizing a monomer component in oil droplets (JP-B-36-9168) in which a compound having a double bond such as an acrylic compound, styrene or vinyl acetate is dissolved in oil droplets and a radical polymerization reaction is allowed to proceed in the presence of a peroxide compound as a catalyst to form an oil-insoluble polymer.
(4) Fusion dispersion cooling methods (British Patents 952,807 and 965,074) in which a stable material which is solid at room temperature, but is made liquid by heating, is used as a shell material for the microcapsules. Wax or thermoplastic resins are used.
(5) Spray drying methods (U.S. Pat. No. 3,111,407 and British Patent 930,422) which utilizes the principle of spray drying and in which an emulsion dispersion composed of solid particles or a liquid in a polymer solution is fed to a spray dryer, the emulsion is atomized in the form of fine particles from a atomizer and the material to be encapsulated is instantaneously surrounded by the polymer.
However, these methods have such problems in the production of the microcapsules that manufacturing processes are complicated and the size of the microcapsules can be hardly controlled. Further, there is a problem that the performance of the capsules is greatly affected by the solvents to be used as core materials. There is a possibility that the property of the diazonium salt is masked by coexisting solvents even when one tries to draw out the characteristics of the diazonium salt present in the capsules.
Accordingly, the present inventors have made studies to prepare the microcapsules, to find out a novel basic substance and to choose the type of supports. As a result, the present invention has been made.